Bolted joints in aircraft transmit large loads and are usually fastened using a plurality of bolts in order to lower the surface pressure between members and increase redundancy. Bolts used in such bolted joints are subjected to loads and shear forces from the parts used to form the holes into which the bolts are inserted. The actual load (load distribution) on each of the bolts of a bolted joint in such instances varies in accordance with the deformation of each bolt and hole. Rigorous predictions of the actual load on each bolt in a bolted joint are therefore difficult to make merely by using finite element method (FEM) analysis, which is usually used for stress analysis. It is also difficult to verify the accuracy of the results of analysis made using the finite element method. Designs tending to extreme safety have therefore generally been implemented wherein the load on each bolt is significantly lower than the allowable yield strength yield strength per bolt. Breaking tests must be performed on the actual mechanical elements of the bolted joint in order to determine the load on each bolt of a bolted joint.
The analytically optimal design of bolted joints, which transmit large loads in aircraft and the like, has thus been difficult. If the actual load distribution of the bolts was known, analysis of the actual load in a bolted joint would be simplified, the precision of designs for bolted joints would be increased, and optimal designs that are lighter and more reliable than conventional designs could be created. So far, however, no precise method has yet emerged for directly measuring bolt load distribution; the only method has involved indirectly measuring the load distribution of the bolts by attaching strain gauges to the perimeter of the bolt holes and measuring strain at those locations.
A technique has been conceived in order to solve such problems, wherein the shear load of a bolt is measured by embedding an optical fiber sensor within the solid shaft of the bolt (see JP-A-2004-212210, for example). This method involves detecting the amount of axial strain of the solid shaft via the optical fiber sensor and measuring the shear stress acting on the solid shaft based on the detected amount of axial strain and the Poisson effect.
A description of a technique for precision measurement using optical fiber sensors is disclosed in the literature (Ishikawa, S., Technique for precision measurement using optical fiber gratings. Applied Physics 2000, 69.6, 648-654, Jun. 10, 2000).
In the method disclosed in the aforementioned patent document, the amount of axial strain observed by the optical fiber sensor must be converted to shear stress based on the Poisson effect in order to determine the shear load on the bolt. The optical fiber sensor must therefore be fixed within the bolt so that the strain of the bolt accurately corresponds to the strain of the optical fiber sensor within the bolt in order to accurately measure the amount of strain in the direction of the solid shaft of the bolt. Fixing the optical fiber sensor in this manner has been extremely difficult.
Accordingly, a demand has arisen for establishing a method for measuring shear load of a fastening tool whereby the shear load on the fastening tool is directly measured based on optical characteristics detected by an optical fiber sensor provided to a location in proximity to the surface of the fastening tool, and the measurement precision can be rapidly enhanced.